With reference to the schematic illustrations of FIGS. 1-2, prior art planetary gear trains 10 include a series of planet gears 12, a sun gear 14, an annular, or ring, gear 16, and a carrier 18. The sun gear and the annular gear are coaxially aligned, with the planet gears being spaced around and meshed between the sun gear and the annular gear. The carrier interconnects the rotational axes of the planet gears. One of the sun gear and the annular gear is grounded, or fixed, with the other of the sun gear and the annular gear being freely rotatable, or unfixed. Accordingly, with reference to FIG. 1, when the sun gear is fixed, a rotational input of the annular gear results in a rotational output of the carrier, and vice versa. With reference to FIG. 2, when the annular gear is fixed, a rotational input of the sun gear results in a rotation output of the carrier, and vice versa. In both configurations and as schematically indicated in FIGS. 1-2, the rotational direction of the input is the same as the rotational direction of the output.
Planetary gear trains are used in a variety of applications to transmit a rotational input having a frequency of rotation (e.g., rotations per minute, or RPM) and torque to a rotational output having a different frequency of rotation and/or torque. When compared to other configurations of transmissions, planetary gear trains typically provide the advantages of a coaxial input and output, a compact and radially symmetrical design, and a high efficiency of energy transfer (i.e., low frictional losses); however, significant forces are required to be transferred between the planet gears and the carrier, resulting in heavy bearing assemblies being required. Moreover, regular maintenance and lubrication of the bearing assemblies are required.
In applications associated with rotorcraft, such as the transmission of a high RPM and low torque engine input to a low RPM and high torque rotor output with a reduction ratio on the order of 60:1, the weight and maintenance of the transmission components can be significant. Existing rotorcraft transmissions rely on a high final drive ratio to reduce or eliminate intermediate reduction stages, which minimizes overall transmission system weight. However, prior art transmissions that utilize planetary gear trains rely on large diameter planet gears to achieve the desired high final drive ratio, and because force is transferred to the carrier by the center of the planet gears via bearing assemblies, the mechanical advantage of the transmission is limited by the diameter of the planet gears. As such, larger planet gears are needed to achieve higher reduction ratios. Larger planet gears limit the number of planet gears that can be used, increase system weight, increase system envelope, and reduce load sharing across multiple planet gears. Accordingly, there is a need for lighter weight, lower maintenance, larger reduction ratios, and smaller envelopes associated with planetary gear trains, such as in applications associated with rotorcraft.